Atrioventricular valves of the mouse: II. Light and transmission electron microscopy

Background: Mouse atrioventricular(AV) valves present a number of conspicuous morphologic differences with human AV valves. Given the existence of these differences, it is important to know the structural oranization of mouse AV valves. Since the mouse is often considered to be good animal model for developmental and anatomical studies, the presence of significant difference in structure may render comparative studies difficult. In addition, we wished to learn about the existence of structural changes in the mouse AV valves with age. Methods:The structural organization of mouse AV valves from 21 days to 1 year of age was studied by polarizing microscopy and by conventional light and transmission electron microscopy. Results:Polarizing microscopy reveals the presence of a system of birefringent fibers that consist of collagen bundles that organize like tendons. The spatial organization of these fibers is different in the two AV valves, reflecting differences in the anatomy of the entire valvular complex. Interstitial cells (IC) are of two different phenotypes: some are typical fibroblast, while some others share smooth muscle cell characteristics. In addition, small areas of fibrocartilage are also observed. The compactness and thickness of the collagen bundles increase with age. Also with age, the basement membranes become thickened or multilayered, and matrix vesicles and deposits of amyloid P can be observed. Conclusions:The collagenous birefringent fibers from an internal skeleton that should transmit the cycling stress evenly over the entire leaflets. IC should help to maintain the structure and deformability of the valve tissue and appear actively involved in the synthesis and renewal of extracellular material. The cartilagionous foci appear to be a normal component of the valve tissue. The Structural changes observed in old animals appear to be related to the degenerative processes which take place in normal valvular tissues with age. Despite the structural differeces, age changes appear to be similar in the AV valves of mouse and man. © 1995 Wiley-Liss, Inc.